This application is based on Japanese Patent Application No. 2002-47908 filed on Feb. 25, 2002, contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an emission control apparatus for engine, specifically to an air-fuel ratio control after a lean air-fuel ratio has continued longer than a predetermined period, especially resuming from a fuel cut operation.
2. Description of Related Art
Heretofore there has been known a technique wherein when an accelerator pedal is released by a driver during operation of an internal combustion engine, a fuel injection control is stopped or significantly decreased to reduce the amount of fuel consumed on condition that the engine speed is higher than a predetermined engine speed. This kind of control is hereinafter referred to as a fuel cut operation or fuel cut. It is generally known that if fuel cut is performed during operation of an internal combustion engine, the amount of oxygen capable of being occluded by a catalyst, e.g., a three-way catalyst, reaches saturation, the catalyst being provided in an exhaust passage of the internal combustion engine for the purification of exhaust gas.
A purification rate of a three-way catalyst indicates a maximum exhaust gas purifying characteristic in the vicinity of a stoichiometric air-fuel ratio. Therefore, there arises an inconvenience such that, even if fuel is fed so as to give a stoichiometric air-fuel ratio after the return from fuel cut, an air-fuel ratio after passing through the three-way catalyst becomes lean with oxygen occluded by the same catalyst.
As techniques for eliminating such an inconvenience there have been proposed a technique disclosed in Japanese Patent No. 2604840 and a technique disclosed in JP-A-8-193537. These techniques employ a system configuration comprising a catalytic converter disposed in an exhaust passage of an engine and a sensor, e.g., an oxygen sensor, disposed downstream of the catalytic converter to detect an oxygen concentration of exhaust gas discharged from an engine.
According to the technique disclosed in Japanese Patent No. 264840, the amount of fuel injected by an injector is increased, or enriched, by a preset amount for prompt consumption of oxygen which has been occluded by the catalytic converter after the return from fuel cut. When the output of the oxygen sensor disposed downstream of the catalytic converter has become rich, the increase, or enriching, of the amount of fuel injected is stopped assuming that the oxygen occluded by the catalytic converter has been consumed.
The system configuration according to the technique disclosed in the JP-A-8-193537 is further provided with a linear A/F sensor for detecting an air-fuel ratio of exhaust gas, the linear A/F sensor being positioned in front of the catalytic converter disposed on the engine side. In such a system, for the consumption of oxygen occluded by the catalyst after the return from fuel cut, the amount of fuel injected by the injector is increased so that an output value of the linear A/F sensor becomes a desired value. According to the technique in question, first in fuel cut, the amount of oxygen occluded by the catalytic converter is estimated. Hereinafter, the amount of oxygen occluded is referred to as an occluded oxygen quantity. Then, at the time of increasing the amount of fuel injected after the return from fuel cut, there is calculated a deoccluded oxygen quantity based on enriching of the air-fuel ratio relative to the estimated occluded oxygen quantity, and the increase of the injected fuel quantity is stopped when the occluded oxygen quantity has reached a level not requiring any further consumption of oxygen.
In the above system configuration, the number of the catalytic converter disposed in the engine exhaust passage is one. But recently, for the purpose of diminishing the emission when an engine is started in the cold, it has been known that a catalytic converter smaller in capacity than the conventional catalytic converter which permits quick warm-up of catalyst is disposed upstream of the exhaust passage. That is, there has been known a system which is provided in the engine exhaust gas passage with a linear A/F sensor, an upstream-side catalyst small in capacity, an oxygen sensor, and a downstream-side catalyst larger in capacity than the upstream-side catalyst, successively from the upstream side.
However, if the foregoing techniques disclosed in Japanese Patent No. 2604840 and the JP-A-8-193537 are applied to such a system, there is a fear that the following inconvenience may occur.
According to the technique disclosed in Japanese Patent No. 2604840, a stop timing of the increase of the fuel injection quantity is determined by the oxygen sensor disposed downstream of catalyst, so in a system not provided with an oxygen sensor downstream of a downstream-side catalyst, it is impossible to determine a stop timing of the increase of the fuel injection quantity. Consequently, there sometimes is a case where a return is made to an ordinary feedback control in a state in which oxygen occluded by the downstream-side catalyst is not consumed to a sufficient degree. Therefore, the increase of the fuel injection quantity is not performed thereafter and it takes time for consumption of the oxygen occluded by the downstream-side catalyst. If the increase of the fuel injection quantity is performed in an actually completely consumed state of the oxygen occluded by the downstream-side catalyst, a rich gas will be released to the atmosphere, with a consequent likelihood of deteriorated emission.
On the other hand, according to the technique disclosed in the JP-A-8-193537, the amount of oxygen occluded in the catalytic converter is estimated. Therefore, it is here assumed that the amount of oxygen occluded by two catalytic converters is estimated and that an increase of the fuel injection quantity is executed on the basis of the estimated value. In the JP-A-8-193537, it is described that an increase of the fuel injection quantity is executed by setting the air-fuel ratio to a value richer by 0.5% to 2.0% than a stoichiometric air-fuel ratio.
However, even if an increase of the fuel injection quantity is set to a 0.5% richer value in terms of air-fuel ratio, it is likely that a long time will be required for the consumption of oxygen occluded by the catalytic converter, making a quick return to the ordinary feedback control impossible. A description will now be given of the case where an increase of the fuel injection quantity is set to a 2.0% richer value in terms of air-fuel ratio. Also in this case, since the amount of oxygen occluded by the catalytic converter is an estimated value, there is the possibility that a 2.0% richer exhaust gas will be released to the atmosphere despite the actual consumption of oxygen, that is, the emission will be deteriorated.
Accordingly, it is an object of the present invention to provide an emission control apparatus for engine capable of rapidly consuming oxygen occluded by a catalytic converter and diminishing emission released to the atmosphere even if an estimated value of the amount of oxygen occluded is deviated from an actual value.
For achieving the above-mentioned object, according to a first aspect of the present invention, an emission control apparatus for engine is applied to an engine control system that has a fuel supply stop means for stopping the supply of fuel injected by a fuel injection valve during operation of the engine. The emission control apparatus comprises a first occluded oxygen quantity estimating means for estimating a total amount of oxygen occluded by an upstream-side catalyst and oxygen occluded by a downstream-side catalyst, a first air-fuel ratio enriching means for enriching the air-fuel ratio of exhaust gas when a return is made from the state in which the supply of fuel is stopped by the fuel supply stop means, and a second air-fuel ratio enriching means which, upon lapse of a first predetermined period after execution of the enriching operation of the first air-fuel ratio enriching means, sets the air-fuel ratio of the exhaust gas to a rich ratio smaller than the degree of richness set by the first air-fuel ratio enriching means. The air-fuel ratio enriching operation of the second air-fuel ratio enriching means is stopped when the total amount of oxygen occluded in both upstream-side catalyst and downstream-side catalyst, which is estimated by the first occluded oxygen quantity estimating means, has become smaller than a predetermined value.
With this construction, for example in a state in which a large amount of oxygen is occluded in both upstream-side catalyst and downstream-side catalyst by fuel cut, the oxygen occluded by both catalytic converters is consumed rapidly by the first air-fuel ratio enriching means. Then, after the lapse of the first predetermined period, the oxygen occluded by both upstream-side catalyst and downstream-side catalyst is consumed by the second air-fuel ratio enriching means which is smaller in the degree of richness than the first air-fuel ratio enriching means, and when the occluded oxygen quantity estimated by the first occluded oxygen quantity estimating means has become smaller than the estimated value, the air-fuel ratio enriching operation of the second air-fuel ratio enriching means is stopped.
Therefore, after the lapse of the first predetermined period, the air-fuel ratio of the mixture fed into the exhaust passage is enriched constantly by the second air-fuel ratio enriching means, so even if an estimated total amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst is deviated from an actual value, it is possible to suppress the influence on the emission because the degree of richness is smaller than in the first air-fuel ratio enriching means.
Moreover, before the enriching operation of the second air-fuel ratio enriching means is executed, there is performed an air-fuel ratio enriching operation by the first air-fuel ratio enriching means, so that oxygen can be consumed in a short time in comparison with the case where the oxygen occluded by both upstream-side catalyst and downstream-side catalyst is consumed at an air-fuel ratio of a small richness degree.
By enriching the air-fuel ratio after the return from fuel cut there occurs a phenomenon that first the oxygen occluded by the upstream-side catalyst is consumed, followed by consumption of the oxygen occluded by the downstream-side catalyst. With such a phenomenon taken into account, since the first and second air-fuel ratio enriching means are switched from one to the other after the lapse of the predetermined period, there is the possibility that an air-fuel ratio enriching operation will be carried out by the first air-fuel ratio enriching means irrespective of the oxygen in the upstream-side catalyst having been consumed.
Consequently, the upstream-side catalyst is likely to assume a rich condition and there is a fear that a smooth return to feedback control may be impossible.
In this connection, according to an embodiment of the present invention, if it is determined that the first predetermined period has elapsed when an air-fuel ratio detected by an oxygen sensor exceeds a second predetermined value, it is possible to effect switching from the air-fuel ratio enriching operation of the first air-fuel ratio enriching means to that of the second air-fuel ratio enriching means when the oxygen occluded by the upstream-side catalyst has been consumed. The air-fuel ratio may be indicated by an output corresponding to an oxygen concentration.
With this construction, it is possible to determine that the oxygen occluded by the upstream-side catalyst has been consumed sufficiently by the first air-fuel ratio enriching means after the return from fuel cut, and after this determination it is possible to effect switching to the second air-fuel ratio enriching means. That is, it is possible to diminish the richness degree of the exhaust gas fed to the upstream-side catalyst at the time of return to a normal control such as feedback control and hence possible to effect a smooth return to the normal control after the end of air-fuel ratio control made by the second air-fuel ratio enriching means.
According to an embodiment of the present invention, when the occluded oxygen quantity estimated by the first occluded oxygen quantity estimating means is smaller than a third predetermined value, it is determined that the first predetermined period has elapsed. That is, by setting the third predetermined value for determining an occluded oxygen quantity to a value indicating that the oxygen occluded by the upstream-side catalyst has been consumed, there can be obtained a similar advantage described above.
According to an embodiment of the present invention, while the supply of fuel from the fuel injection valve is stopped by the fuel supply stop means, the first occluded oxygen quantity estimating means estimates the amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst on the basis of the amount of intake air. Since the amount of oxygen fed to the catalysts during fuel cut is proportional to the amount of intake air, the amount of oxygen occluded by both upstream- and downstream-side catalysts can be estimated accurately on the basis of the amount of intake air.
According to an embodiment of the present invention, as the amount of oxygen estimated by the first occluded oxygen quantity estimating means, there may be estimated the amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst on the basis of a period during which the supply of fuel from the injection valve is stopped by the fuel supply stop means. This permits the amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst to be estimated in a simpler manner than described above.
According to an embodiment of the present invention, an emission control apparatus for engine further comprises a determining means for determining that a leaner state of the exhaust gas air-fuel ratio detected by the first air-fuel ratio detecting means than a fourth predetermined value has continued for a second predetermined period.
In this case, the first air-fuel ratio enriching means enriches the exhaust gas air-fuel ratio when it is determined by the determining means that a leaner state of the exhaust gas air-fuel ratio than the fourth predetermined value has continued for the second predetermined period and when the exhaust gas air-fuel ratio has exceeded a fifth predetermined value richer than the fourth predetermined value from the leaner state than the fourth predetermined value. The second air-fuel ratio enriching means, upon lapse of a predetermined period after the execution of the enriching operation of the first air-fuel ratio enriching means, sets the exhaust gas air-fuel ratio to a rich value smaller than the degree of richness set by the first air-fuel ratio enriching means. The air-fuel ratio enriching operation of the second air-fuel ratio enriching means is stopped when the total amount of oxygen occluded in both upstream-side catalyst and downstream-side catalyst which is estimated by the occluded oxygen quantity estimating means has become smaller than the predetermined value.
Even when the air-fuel ratio controlled for an internal combustion engine is lean, oxygen is occluded by both upstream-side catalyst and downstream-side catalyst. Therefore, by determining such conditions as permit oxygen to be occluded by both upstream-side and downstream-side catalyst and by using the first and second air-fuel ratio enriching means, it is possible to obtain a similar advantage as described above even in any other case of oxygen being occluded by both upstream-side catalyst and downstream-side catalyst than during fuel cut.
According to an embodiment of the present invention, an emission control apparatus for engine further comprises a second occluded oxygen quantity estimating means for estimating the amount of oxygen occluded by the downstream-side catalyst, and wherein the air-fuel ratio enriching operation of the second air-fuel ratio enriching means is stopped when the amount of oxygen estimated by the second occluded oxygen quantity estimating means has become smaller than the first predetermined value.
With this construction, since the amount of oxygen occluded by the downstream-side catalyst can be estimated, it is possible to stop the enriching operation of the second air-fuel ratio enriching means when the oxygen occluded by the downstream-side catalyst has been consumed.
According to an embodiment of the present invention, an emission control apparatus for engine further comprises a deoccluded oxygen quantity computing means for computing the amount of oxygen which is deoccluded from the upstream-side catalyst by the first air-fuel ratio enriching means, and wherein on the basis of the deoccluded oxygen quantity from the upstream-side catalyst computed by the deoccluded oxygen quantity computing means, the second occluded oxygen quantity estimating means estimates the amount of oxygen occluded by the downstream-side catalyst.
The amount of oxygen deoccluded by the first air-fuel ratio enriching means corresponds to the amount of oxygen occluded by the upstream-side catalyst. The upstream-side catalyst and downstream-side catalyst are different in point of capacity, but their occluded oxygen quantities are correlated with each other. Therefore, the amount of oxygen occluded by the downstream-side catalyst can be estimated with high accuracy on the basis of the deoccluded oxygen quantity computed.
According to an embodiment of the present invention, the first occluded oxygen quantity estimating means compares the amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst which amount is obtained by estimation, with a saturated amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst. The first occluded oxygen quantity estimating means sets the amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst to the stored value in response to the result of comparing the estimated value with the stored value.
This permits an occluded oxygen quantity to be estimated with high accuracy even when the amount of oxygen occluded by both upstream-side catalyst and downstream-side catalyst reaches saturation.
If the stored value of the saturated amount of occluded oxygen were deviated from the actual saturated amount of occluded oxygen, enriching would be performed by the second air-fuel ratio enriching means in an actually consumed state of oxygen occluded by both upstream-side catalyst and downstream-side catalyst, or the second air-fuel ratio enriching means might be stopped in an unconsumed state of oxygen.
A description will now be given about such a case. In this embodiment, as noted earlier, the saturated amount of oxygen occluded by the upstream-side catalyst and that occluded by the downstream-side catalyst are correlated with each other. Therefore, each of such saturated amounts can be obtained on the basis of the stored value. Further, the saturated amount of oxygen occluded by the upstream-side catalyst corresponds to the amount of oxygen deoccluded from the same catalyst. Since the amount of oxygen deoccluded from the upstream-side catalyst can be determined from the state in which the output of the oxygen sensor has reached a predetermined degree of richness, the saturated amount of oxygen occluded by the upstream-side catalyst can be determined from the deoccluded oxygen quantity.
According to an embodiment of the present invention, the stored value is corrected on the basis of the deoccluded oxygen quantity from the upstream-side catalyst computed by the deoccluded oxygen quantity computing means. With this construction, even if the stored value of the saturated amount of oxygen is deviated from the actual saturated amount of oxygen, it can be corrected on the basis of the deoccluded oxygen quantity from the upstream-side catalyst computed by the deoccluded oxygen quantity computing means.